Vegetative propagation may be accomplished by natural (natural vegetative propagation) as well as artificial (artificial vegetative propagation) means. Since plants resulting from vegetative propagation are produced asexually from a single parent plant, they are genetic clones of the parent plant. This can have advantages and disadvantages. One advantage of vegetative propagation is that plants with traits that are favorable for a particular environment are repeatedly reproduced. Commercial crop growers employing artificial vegetative propagation techniques can ensure that favorable traits and product quality are maintained. A major disadvantage of vegetative propagation is that this process does not allow for genetic variation. The plants are genetically identical and are all susceptible to the same plant viruses and diseases that can destroy entire crops. Natural vegetative propagation involves the development of a new plant from parts of a single mature plant. The new plants grow and develop naturally without human intervention. An important ability that is key to enabling vegetative propagation in plants is the ability to develop adventitious roots. These are roots that arise from plant structures other than the root, such as stems or leaves. Through the formation of adventitious roots, new plants may develop from extensions of the stems, roots, or leaves of a parent plant. Modified stems are most often the source of vegetative propagation in many plants. Vegetative plant structures that arise from plant stems include rhizomes, runners, bulbs, tubers, corms, and buds. Vegetative structures emanating from roots include buds and tubers. Plantlets are vegetative structures that emerge from plant leaves. Vegetative propagation may occur naturally through the development of rhizomes. Rhizomes are modified stems that typically grow horizontally along the ground surface or underground. Rhizomes are storage sites for substances such as proteins and starches. As rhizomes extend, roots and shoots may arise along certain intervals of the rhizome and develop into new plants. Certain grasses, lilies, irises, and orchids propagate in this manner. Edible plant rhizomes include ginger and tumeric.